Abstract: The existing of a little gas or vapour content has great influence upon the hydraulic transient of pipe flow. In this paper, the transient processes of pipe flow with different initial gas holdup are researched by means of experiments, and several distinct pressure characteristics are observed. The effects of initial gas holdup on the hydraulic transient are fairly large. The less the initial void fraction (α₀), the sharper the pressure responds. When α₀ increases, the peak value of the response will lower, and the attenuation of transient press is more rapid. When α₀ < 0.0001, the pressure peak varies with α₀ not noticeably. Even it can be higher than that predicted when α₀=0.